“What's a writer to do when a minor character that they created for a show suddenly becomes much more popular than the other members of the cast? Why, re-write them as a main character, of course! This is that character[…]other characters in the cast might be Demoted to Extra[2] due to their reduced roles and screentime[…]the popularity of the character has to have influenced the direction of the narrative and put them into a major role, or led to the creation of new content specifically for them.”
I’d say there’s also shades of Spotlight-Stealing Squad[3].
[1] https://tvtropes.org/pmwiki/pmwiki.php/Main/BreakoutCharacte...
[2] https://tvtropes.org/pmwiki/pmwiki.php/Main/DemotedToExtra
[3] https://tvtropes.org/pmwiki/pmwiki.php/Main/SpotlightStealin...